The present disclosure relates to holograms, methods of making and using holograms, and more particularly to polychromatic holograms. Articles incorporating the polychromatic holograms are also disclosed.
Volume holograms are an increasingly popular mechanism for the authentication of genuine articles, whether it is for security purposes or for brand protection. The use of volume holograms for these purposes is driven primarily by the relative difficulty with which they can be duplicated. Volume holograms are created by interfering two coherent beams of light to create an interference pattern and storing that pattern in a holographic recording medium. Information or imagery can be stored in a hologram by imparting the data or image to one of the two coherent beams prior to their interference. The hologram can be read out by illuminating it with a beam of light matching the geometry and wavelength of either of the two original beams used to create the hologram and any data or images stored in the hologram will be displayed. Typically, holograms can be read out by other light wavelengths that satisfy the Bragg diffraction condition at suitably adjusted angles of incidence and divergence. As a result of the complex methods required to record holograms, their use for authentication can be seen on articles such as credit cards, software, passports, clothing, and the like.
The most common types of volume holograms are transmission holograms and reflection holograms. To form any volume hologram, two light beams are used. One beam, known as the signal beam, carries the image information to be encoded in the hologram. The second beam can be a plane wave or a convergent/divergent beam with no information, also known as the reference beam. The object (or signal) beam and the reference beam generate an interference pattern, which is recorded in the form of a diffraction grating within the holographic medium. A transmission hologram is created when both object and reference beams are incident on the holographic medium from the same side, and is so called because in viewing the hologram, the light must pass through the holographic material to the viewer.
On the other hand, during the recording of a reflection hologram, the reference beam and the object beam illuminate the holographic medium from opposite sides, and the hologram is viewed from the same side of the material as it is illuminated. Generally, a reflection hologram only reflects light within a narrow band of wavelengths around the writing wavelength. Because of this, the holographic image created by a reflection hologram tends to appear monochromatic. The interference fringes in the holographic material are formed by standing waves generated when the two beams, traveling in opposite directions, interact. The fringes formed are in layers substantially parallel to the surface of the film. Generally, reflection holograms will only reflect wavelengths that are the same as or close to the fringe spacing of the hologram, which is determined by the writing wavelength (λ). Therefore, for a particular writing wavelength (λ), the hologram will appear monochromatic and reflect only wavelengths close to λ.
Considerable efforts have been made to develop multi-color reflection holograms. In a conventional approach to multi-color reflection holography, beams of coherent light in each of the primary (additive) colors (red, green, and blue) are used to record distinct holograms in the holographic media plate. In practice, registration of the three images is very difficult, particularly for large plates, insofar as the focus and/or magnification of each beam is dependent on its wavelength; good registration requires careful angular adjustment of the beams. The need for additional reference light sources (e.g., a blue light laser and a green light laser, as well as a red light laser) also adds a substantial complexity and cost to the system. Moreover, the recording medium (e.g., a photographic emulsion or photopolymer) typically exhibits different sensitivities at different wavelengths, dictating careful control of the exposure time and intensity of each beam as well. Also, many types of holographic recording media are not readily prepared with sensitivities to each of the primary light colors. For example, holographic recording media using photochemically active dyes may impart unwanted coloration (through absorbance) that may aesthetically interfere with the color image produced by the holographic diffraction grating. Other techniques for expanding the differential between the writing wavelength and the viewing wavelength have involved physical deformation of the recording medium such as by swelling with a solvent prior to exposure, followed by drying and shrinking the medium after exposure. However, such techniques increase the complexity of the recording process and may not be feasible with many types of recording media.
Thus, there remains a need for improved methods of making reflection holograms. More particularly, there remains a need for simpler, more cost effective methods of making complex, e.g., multicolor, holograms.